1. Field of the Invention
The present invention relates to an improved span bolster arrangement for connecting pairs of railway trucks to a vehicle chassis in such a manner as to minimize weight transfer from one truck to the other when the axles of the trucks are exerting tractive or braking effort.
2. Description of the Prior Art
When it is desired to increase the number of powered axles in a locomotive truck to more than three, two possibilities are immediately evident. One solution is to provide a longer rigid wheel base, with four or more axles. However, a longer rigid wheel base necessarily develops higher flange forces when negotiating curves, and can also be subject to axle load changes when negotiating vertical curves, depending upon the suspension arrangement. The other solution is to utilize sub-bogie or truck arrangements, the sub-bogie or truck arrangements being connected by a structure commonly referred to as a span bolster. In particular, a span bolster arrangement is a sub-frame structure sitting on top of two trucks with the effective weight of one end of the locomotive frame or railway car being supported by the span bolster. In known span bolster arrangements, the effective weight of one end of the locomotive frame or railway car is applied to the span bolster structure at a location approximately mid-way between the two trucks. This arrangement is commonly used with a locomotive frame supported by four 2-axle trucks, such that the locomotive railway car has eight axles altogether. Other less common truck arrangements for supporting locomotives or railway cars include the use of four 3-axle trucks with a span bolster interconnecting the two pairs of 3-axle trucks. In this arrangement, the locomotive or railway car is supported by twelve axles altogether. Additionally, a ten-axle arrangement can be utilized, this arrangement having two 2-axle trucks interconnected by a span bolster and two 3-axle trucks interconnected by a span bolster. Other combinations of truck arrangements can also be provided, including arrangements where a span bolster interconnects a 2-axle truck and a 3-axle truck.
The use of a span bolster arrangement provides a very flexible support system for a locomotive or railway car insofar as the ability of the locomotive or railway car to negotiate lateral and vertical curves. Further, the end of the span bolster structure adjacent the end of the locomotive or railway car can be adapted to support a coupler arrangement, the coupler arrangement being designed to interconnect one locomotive or railway car to another. However, for the purposes of the present invention, the couplers must be positioned directly on the locomotive frame in order to realize the zero weight transfer potential from one truck to another.
It is known in the prior art to provide a truck assembly adapted to maintain substantially equal loading on the axles of a 2-axle truck. In this regard, U.S. Pat. No. 3,713,397, issued Jan. 30th, 1973 to Parker et al, discloses such an arrangement whereby a 2-axle truck is resiliently suspended on a central post, the resilient suspension permitting rotation of the truck about a horizontal axis located approximately at ground level and in line with the vertical axis of rotation of the truck about the post. Such an arrangement facilitates balancing of loading on the drive wheels of the truck during acceleration or deceleration, thereby improving the overall traction of the locomotive. Further, U.S. Pat. No. 2,954,747 to Hirst et al discloses an arrangement for locating the traction point of a bogie or truck having an uneven number of pairs of wheels beneath the central axle of the truck. By utilizing such an arrangement, the bolster is eliminated such that the weight and complexity of the suspension is correspondingly reduced.
Other low weight transfer trucks with which the present invention might be utilized have been described in U.S. Pat. Nos. 3,563,185 (Gen. Steel, 2-arm arrangement) 3,547,046 (Gen. Steel 4-arm arrangement). In addition to U.S. Pat. No. 2,954,747, a further 3-axle truck low weight transfer arrangement is described in U.S. Pat. No. 3,693,553 (Gen. Steel).
Each of the prior art patents referred to recognize the advantage of utilizing zero weight transfer trucks by locating the traction point, that is, the point of application of the resultant longitudinal force acting between the truck and the body of the railway vehicle, at or adjacent to rail level. Since any structure at rail level would foul trackwork, the above solutions for establishing the traction point adjacent rail level do so by establishing a virtual, or non-real, traction point at or adjacent to rail level. However, in known arrangements utilizing span bolsters mounted on trucks with the weight of one end of the locomotive chassis mounted on the span bolster being applied to the span bolster at a location midway between the trucks, weight transfer occurs from one truck to the other when axles of the trucks are exerting tractive or braking effort. The transfer of weight from one truck to another results in the wheels of the truck from which the load is being transferred losing their adhesion with the rails, such that the traction motors connected to the axles of the truck do not effectively transfer their power to the rails.
Apart from providing a span bolster construction which minimizes weight transfer from one truck to another, there are specific design constraints with respect to the construction of span bolsters. These design constraints include the following:
(a) The bolster must receive the effective weight of the portion of the chassis supported thereby at a location approximately midway along the length of the span bolster plate in order to permit distribution of the load to the two trucks.
(b) The locomotive chassis must suitably engage the span bolster, and the entire assembly (trucks, span bolster, and chassis) must pass through the clearance diagram of the user railroad; and
(c) Other design considerations with respect to ease of movement, dynamic stability, good maintainability, etc. must be met, as for any other running gear arrangement.
With respect to the design criteria noted above, the major problem of the present span bolsters is to meet the constraint noted under heading (b) above, that is, passing through the given clearance diagram. In particular, where the load of one end of a locomotive chassis or railway car is applied at a location midway between the two trucks, that is, half way along the length of the span bolster, substantial bending moments are developed in the longitudinal members of the span bolster, necessitating the use of a large span bolster structure, such that the resulting locomotive or railway car is usually quite tall and, in certain instances, too tall to meet the clearance diagram requirements of the user railroad.
In addition to minimizing the weight transfer from one truck to another, the present invention proposes to provide a span bolster structure compatible with 2-axle and 3-axle trucks, which minimizes bending moments in the longitudinal members of the span bolster, thereby permitting the use of a smaller span bolster structure. Minimization of the bending moments in the longitudinal members of the span bolster and minimizing load transfer from one truck to another is achieved, in part, by providing a direct load transfer from the chassis, through the span bolster, to the trucks. Thus, to avoid transferring weight from one truck to the other, the span bolster arrangement must have points of attachment to the chassis of the locomotive or railway car that are collinear with the points of attachment of the trucks to the span bolster. To achieve the most effective load path from chassis to trucks, the portion of the weight of the locomotive chassis or railway car supported by each span bolster should be applied equally and directly to each of the trucks or, in effect, the portion of the chassis supported by the span bolster should sit on the two, three or more trucks of the arrangement in the manner of a beam having multiple equal-reaction supports. Although the span bolster is situated between the chassis and the trucks, the load path from the chassis to the trucks is still considered as being direct, since the locations where the portion of the chassis load is supported by the span bolster are substantially collinear or co-planar with locations where the loads are applied by the span bolster, with the chassis mounted thereon, to the trucks, when viewed from the side of the locomotive. In this way, the loads applied to the span bolster at each location along the length thereof are balanced in a vertical direction, whereby bending moments in the longitudinal members of the span bolster are minimized and with proper design care, will be substantially zero.